fix(host-agent): thread device manager into task runner observer/screenshot

create_task_runner() built TaskRunner's observer/screenshot_provider by
calling describe_screen(device_id)/take_screenshot(device_id) without
manager=, so both silently fell back to the process-global DEFAULT_MANAGER
singleton instead of the Host Agent's real, device-populated DeviceManager.
DEFAULT_MANAGER never has any device registered, so every task's first step
raised DeviceNotFoundError even though the console (which does pass
manager=) showed the same device as connected. Deterministic on every task,
independent of process count.

Add regression tests confirming both lambdas now resolve devices via the
configured manager; verified each fails with the original DeviceNotFoundError
symptom when the fix is reverted.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/device-host-agent/tests/test_execution.py

@@ -5,6 +5,8 @@ from pathlib import Path
 import pytest
 
 from device.manager import DeviceManager
+from driver.base import Driver
+from host_agent import execution
 from host_agent.cloud_planner_client import CloudProxyToolCallingClient
 from host_agent.config import HostAgentConfig
 from host_agent.execution import create_execution_factories
@@ -15,6 +17,54 @@ from workflow.runner import WorkflowRunner
 from workflow.store import WorkflowStore
 
 
+class FakeDriver(Driver):
+    def __init__(self) -> None:
+        self.calls: list[tuple[str, tuple[object, ...]]] = []
+
+    def connect(self) -> None:
+        self.calls.append(("connect", ()))
+
+    def disconnect(self) -> None:
+        return None
+
+    def screenshot(self) -> bytes:
+        return b"fake-screenshot-bytes"
+
+    def tap(self, x: float, y: float) -> None:
+        return None
+
+    def swipe(
+        self,
+        start_x: float,
+        start_y: float,
+        end_x: float,
+        end_y: float,
+        duration_ms: int = 500,
+    ) -> None:
+        return None
+
+    def input(self, text: str) -> None:
+        return None
+
+    def launch(self, app_id: str) -> None:
+        return None
+
+    def terminate(self, app_id: str) -> None:
+        return None
+
+    def tree(self):
+        return None
+
+    def home(self) -> None:
+        return None
+
+    def lock(self) -> None:
+        return None
+
+    def unlock(self) -> None:
+        return None
+
+
 def test_execution_factories_compose_existing_runtime_and_workflow(tmp_path) -> None:
     manager = DeviceManager()
     workflow_store = WorkflowStore(tmp_path / "workflows.sqlite3")
@@ -32,6 +82,54 @@ def test_execution_factories_compose_existing_runtime_and_workflow(tmp_path) ->
     assert isinstance(workflow_runner.task_runner_factory(), TaskRunner)
 
 
+def test_created_task_runner_screenshot_provider_uses_configured_manager(
+    tmp_path,
+) -> None:
+    """Regression test: `create_task_runner()` must thread the Host Agent's
+    own `manager` into `screenshot_provider`. Omitting `manager=` makes the
+    tool fall back to the process-global `DEFAULT_MANAGER` singleton, which
+    never has this device registered, so it raises `DeviceNotFoundError` even
+    though the device is connected on the manager actually in use.
+    """
+    manager = DeviceManager()
+    manager.register_device("phone-1", lambda: FakeDriver())
+    manager.connect("phone-1")
+    factories = create_execution_factories(
+        manager, workflow_store=WorkflowStore(tmp_path / "workflows.sqlite3")
+    )
+
+    task_runner = factories.task_runner_factory()
+
+    assert task_runner.screenshot_provider("phone-1") == b"fake-screenshot-bytes"
+
+
+def test_created_task_runner_observer_uses_configured_manager(
+    tmp_path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """Regression test: `create_task_runner()` must thread the Host Agent's
+    own `manager` into `observer` the same way it does for
+    `screenshot_provider` -- see the test above for the failure mode this
+    guards against.
+    """
+    manager = DeviceManager()
+    seen: dict[str, object] = {}
+
+    def fake_describe_screen(device_id, *, manager=None):
+        seen["device_id"] = device_id
+        seen["manager"] = manager
+        return "scene-stub"
+
+    monkeypatch.setattr(execution, "describe_screen", fake_describe_screen)
+    factories = create_execution_factories(
+        manager, workflow_store=WorkflowStore(tmp_path / "workflows.sqlite3")
+    )
+
+    task_runner = factories.task_runner_factory()
+
+    assert task_runner.observer("phone-1") == "scene-stub"
+    assert seen == {"device_id": "phone-1", "manager": manager}
+
+
 def test_created_task_runner_defaults_to_ai_planner(
     tmp_path, monkeypatch: pytest.MonkeyPatch
 ) -> None: